1. Field of Invention
The present invention relates to a method for forming a gate. More particularly, the present invention relates to a method for forming a protective layer to protect a gate during a rapid thermal annealing process.
2. Description of Related Art
Tungsten silicide (WSi.sub.x) is commonly used as a gate conductor in integrated circuits, especially for memory chips, due to its high thermal stability, low resistivity, low contamination levels and good step coverage. The tungsten silicide layer, which is usually formed by low pressure chemical vapor deposition (LPCVD) with dichlorosilane (SiH.sub.2 Cl.sub.2) and tungsten hexafluoride (WF.sub.6) as a gas source, has many advantages--lower fluorine content, improved step coverage, lower post-annealed stress and better adhesion with other materials. Therefore, nowadays, the tungsten silicide layer is formed in the manner described above in some semiconductor processes.
In the process of forming a gate structure, a nitride layer serving as a cap layer of the gate structure is usually formed on a tungsten silicide layer after a doped polysilicon layer and a tungsten silicide layer are formed on a substrate in sequence. The nitride layer is formed by low pressure chemical vapor deposition at about 700-800.degree. C. The LPCVD chamber is a vertical furnace without a load lock, so oxygen contamination can occur during wafer loading.
Since the tungsten silicide layer is in an oxidizing atmosphere at a high temperature, silicon dioxide (SiO.sub.2) is formed during the process of forming the nitride layer as long as the silicon content of the tungsten silicide layer is adequate. However, if the silicon content of the tungsten silicide layer is limited, an abnormal WSi.sub.x oxidation effect occurs. Due to the abnormal WSi.sub.x oxidation effect, WSi.sub.x is decomposed; thus, SiO.sub.2, elemental W, WO.sub.2, WO.sub.3, and other volatile tungsten oxides are formed. Both WO.sub.2 and WO.sub.3 are volatile (the sublimation point for WO.sub.2 is 800.degree. C.) so that a cracking effect and a blistering effect occur in the tungsten silicide layer.
After the gate structure is defined, a rapid thermal annealing process is performed. The rapid thermal annealing process is usually performed in an oxygen atmosphere and a temperature of the rapid thermal annealing process is raised to about 1000.degree. C., quickly. The rapid thermal annealing process oxidizes sidewalls of the gate structure to form an isolation structure which prevents the gate structure from coupling with a contact or other devices. Oxide protrusions are also easily formed on the sidewalls of the tungsten silicide layer during the rapid thermal annealing process. Therefore, some regions of a subsequently formed spacer are thin or the oxide protrusions protrude from the spacer. In a subsequent process of forming a bit line contact hole, a portion of the gate structure under the thin region of the spacer may be exposed during the etching process. As a result, wordline-to-bitline leakage occurs because the gate structure couples with a plug within the bit line contact hole. If these oxide protrusions protrude from the spacer and block the bit line contact hole, the conductive material does not easily fill the bit line contact hole. So, an open bitline contact problem generates.
Another function of the rapid thermal annealing process is to change the crystal structure and grain size of WSi.sub.x, so that sheet resistance of the tungsten silicide layer can be reduced. For example, the crystal structure of WSi.sub.x is transformed into a tetragonal structure by performing the rapid thermal annealing process at 700.degree. C.; the sheet resistance of the tungsten silicide layer is decreased from 30 to 3 ohm/sq. after the rapid thermal annealing process. In addition, the grain size of WSi.sub.x is less than 100 nm. However, an agglomeration effect of the tungsten silicide layer easily occurs during the rapid thermal annealing process, and hence causes a narrow linewidth effect on the gate structure. Because of the agglomeration effect of the tungsten silicide layer, the surface of the tungsten silicide layer becomes rough, and the sheet resistance of the tungsten silicide layer is increased.